On Mon, Jun 1, 2026 at 4:48 AM Lance Yang <[email protected]> wrote: > > > > On 2026/6/1 18:23, David Hildenbrand (Arm) wrote: > > On 6/1/26 11:08, Lance Yang wrote: > >> > >> > >> On 2026/6/1 14:54, David Hildenbrand (Arm) wrote: > >>> On 6/1/26 05:28, Lance Yang wrote: > >>>> > >>>> > >>>> Ah, fair point. > >>>> > >>>> I was mostly worried about arch hooks that walk vma->vm_mm again, rather > >>>> than only using the pte pointer passed in. For example, mips does: > >>> > >>> Right, a re-walk would be the real problem. > >>> > >>>> > >>>> update_mmu_cache_range() > >>>> -> __update_tlb() > >>>> -> pgd_offset(vma->vm_mm, address) > >>>> -> pte_offset_map(...) > >>>> > >>>> and __update_tlb() has this assumption: > >>>> > >>>> /* > >>>> * update_mmu_cache() is called between pte_offset_map_lock() > >>>> * and pte_unmap_unlock(), so we can assume that ptep is not > >>>> * NULL here: and what should be done below if it were NULL? > >>>> */ > >>>> > >>>> So if khugepaged happens to run with current->active_mm == vma->vm_mm > >>>> here, could __update_tlb() hit the none PMD, get NULL from > >>>> pte_offset_map(), and then dereference it? > >>> > >>> Likely yes -- that MIPS code is horrible. And the comment in MIPS code > >>> even spells that out. :( > >>> > >>> Do you know about other code like that, or is MIPS the only one doing a > >>> re-walk and crossing fingers? > >>> > >>>> > >>>> Just wanted to raise it since some arch code may still have assumptions > >>>> like this, and the always-enable-mTHP work is getting closer ... > >>> > >>> Right. I assume set_pte_at() couldn't trigger something similar (re-walk) > >>> in > >>> arch code, > >>> because we simply provide the ptep. update_mmu_cache_range() only > >>> consumes the > >>> pte. > >>> > >>>> > >>>> Probably very very very hard to hit, though :) > >>> > >>> Delaying update_mmu_cache_range() is nasty, as we'd have to make sure that > >>> nobody can interfere in the meantime ... and the PMD lock will not be > >>> sufficient. > >>> > >>> Maybe we could reinstall the page table with the cleared (none) entries > >>> while > >>> still holding the PTL? > >>> > >>> Thinking out loud: > >>> > >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >>> index 5ba298d420b7..e39b750b1e6f 100644 > >>> --- a/mm/khugepaged.c > >>> +++ b/mm/khugepaged.c > >>> @@ -1413,13 +1413,17 @@ static enum scan_result collapse_huge_page(struct > >>> mm_struct *mm, unsigned long s > >>> map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); > >>> } else { > >>> /* > >>> - * set_ptes is called in map_anon_folio_pte_nopf with the > >>> - * pmd_ptl lock still held; this is safe as the PMD is > >>> expected > >>> - * to be none. The pmd entry is then repopulated below. > >>> + * Re-insert the page table with the cleared entries, but > >>> + * hold the PTL, such that no one can mess with the > >>> re-installed > >>> + * page table until we updated the temporarily-cleared > >>> entries > >>> + * through map_anon_folio_pte_nopf(). > >>> */ > >>> - map_anon_folio_pte_nopf(folio, pte, vma, start_addr, / > >>> *uffd_wp=*/ false); > >>> - smp_wmb(); /* make PTEs visible before PMD. See > >>> pmd_install() */ > >> > >> One small thing, I think we should probably keep the smp_wmb(), and just > >> move it before the earlier pmd_populate(). > >> > >> IIUC, the ordering we want is still: > >> > >> clear old PTEs > >> smp_wmb() > >> pmd_populate() > >> > >> so another CPU cannot walk through the re-installed PMD and still observe > >> the old PTEs, right? > > > > There is a smp_wmb() in __folio_mark_uptodate(), that should be sufficient? > > Ah, cool! __folio_mark_uptodate() already does the job :P > > So yeah, no extra smp_wmb() needed here!
are we sure? that folio_mark_uptodate is done before the PTEs are reinstalled. Then we reinstall the PMD right after. Currently separated by the smp_wmb(). I was copying this from other THP code that performs similar PTE/PMD juggling. I can remove it, but I'd rather air on the side of caution with this. > > Cheers, Lance >
